The present invention relates to an injector drive circuit for a fuel injection system comprising a control circuit for calculating the amount of the fuel supplied to the engine as a time width of an electrical pulse and an injector with the valve thereof opened in accordance with the time width of the electrical pulse supplied from the control circuit, or more particularly to an injector drive circuit for the fuel injection system, in which the current supply to the injector is controlled optimally.
An example of the conventional drive circuits of this type is disclosed in Japanese Patent Laid-Open Publication No. 125932/77 corresponding to U.S. Pat. No. 4,180,026. FIG. 1 shows a schematic block diagram showing the same and FIG. 2 shows waveforms of operation of various parts thereof. This example will be explained with reference to FIGS. 1 and 2. When a comparator is supplied with a pulse signal .tau. from a control circuit ECU for calculating the amount of the fuel supplied to the engine as an injection time width as shown in FIG. 2(a), the output of the comparator 1 becomes low level as shown in FIG. 2(b). This signal is converted into high level at a voltage converter circuit 2 thereby to turn on a transistor Tr1. A current begins to flow in an injector 130, so that the emitter voltage of the transistor Tr1 begins to increase slowly as shown in FIG. 2(e) by the particular current and the resistor R1 connected between the emitter and the earth. When this voltage exceeds the first reference voltage V1 of the comparator 3, the comparator 3 reaches a high level as shown in FIG. 2(c). This high-level signal is applied to the voltage converter circuit 2 in a circuit configuration to turn off the transistor Tr1. Upon the turning off of the transistor Tr1, the current stops flowing in the injector 130, that is, the current more than the peak value corresponding to the first reference voltage V1 does not flow in the injector 130. Then the emitter voltage of the transistor Tr1 is reduced to low level, and the output of the comparator 3 is also reduced to low level again as shown in FIG. 2(c). In view of the fact that signal .tau. from the control circuit ECU is still at high level and that the output of the comparator 1 is at low level, however, the current begins to flow again in the injector 130 as soon as the output of the comparator 3 is reduced to low level. The circuit is so configured that at the time point when the emitter voltage of the transistor Tr1 exceeds the first reference voltage V1 of the comparator 3 and the comparator 3 becomes high in level, the output of the bistable multivibrator 4 is reduced to low level as shown in FIG. 2(d), whereby the reference voltage of the comparator 3 is changed to the second reference voltage V2 set lower than the voltage V1. For this reason, when the current begins to flow again in the injector 130 and the emitter voltage of the transistor Tr1 increases as shown in FIG. 2(e), the output of the comparator 3 reaches a high level as shown in FIG. 2(c) at the time point when the emitter voltage of the transistor Tr1 exceeds the second reference voltage V2 of the comparator 3, so that the transistor Tr1 is turned off and the emitter voltage of the transistor Tr1 is reduced to low level, thus reducing the output of the comparator 3 to low level again. In this way, the current flowing in the injector 130 is maintained at a holding current corresponding to the second reference voltage V2. This holding current is small as compared with the peak current and substantially constant. When the signal .tau. applied to the comparator 1 is reduced from high level to low level, the transistor Tr1 is turned off followed by the signal .tau. increasing from low level to high level, in preparation for the next cycle of similar operation. A delay circuit 5 is for delaying somewhat the timing of reduction of the bistable multivibrator 4 to low level, so that the timing of the turning on of the transistor Tr2 is delayed and thus the injector valve opening time is advanced by use of the reverse electromotive force generated by the turning off of the injector 130 when the peak current of the injector 130 is reached. The flyback control section 6 is for absorbing the reverse electromotive force generated by the turning on and off of the transistor Tr1 in the operation range of the holding current after the decrease of the output of the bistable multivibrator 4 to low level and the similar decrease of the output of the delay circuit 5 to low level, thus preventing the injector from being easily closed by the reverse electromotive force.
The conventional injector drive circuit of this type has the following disadvantages:
(1) The circuit is complicated.
(2) The peak current or the holding current (that is, the first and the second reference voltages V1 and V2 of the comparator 3) cannot be adjusted independently, and the adjustment takes a long time.
(3) If the frequency of setting the holding current by feedback is increased, the effect of the malfunction of other circuits cannot be ignored. If it is reduced, on the other hand, the variation in the holding current is enlarged, and when the system is mounted in a car, the injector is likely to open or close by the vibrations of the car.